November 29, 2008

This is just a rough mock up of what I have been thinking about for a single space mixer. It's 6 channels, each with voltage controlled and manual level and panning. There is a stereo, unity gain expansion input and a mono output. A mute switch on each channel. The 3 channels to the left pan right with higher voltage and the 3 channels to the right do the reverse. Panning is equal powered, so there is no dead zone in the middle. I think it fits the bill.

November 26, 2008

With the release of the Buchla 206e, there's been some chatter about mixers. Evidently some people wanted to have the patch storage, but not the MIDI control for their 200e. But people more like me said they would like a 1 space all analog mixer for their system. This got me thinking, "what have the mixers been so far?"

206 "elegant" 2 x 3 channel mixers that are also summed to operate as a mono 6 channel one. It has mute and monitor switches on all channels.

205 2 x 5 in 4 out matrix mixers that also sum to make a simple 10 in 4 out one. Monitor switches on each input channel. Not exactly small at 3 panel spaces.

The 226 and 227 are quad interfaces that for the purpose of this discussion are irrelevant.

I think it would be nice to have a 6 channel, stereo mixer with CV controlled level and pan on each channel. I love sliders too. I don't know if anyone besides Grant uses the monitor system, so that might be eliminated.

The 207 uses a pair of Vactrols to do the panning, but because of the way the equal power curve is implimented, I don't think I could just send the level CV into the bottom of the differential pair. I could put basically a lopass gate in gate mode on each channel before the panner. The parts count could add up REALLY fast. That's 18 Vactrols! That's a lot of board space.

I'm thinking about solutions that are more elegant, like how the Serge Universal Audio Processor works. It's just 2 VCAs per channel that are controlled for the gain and panning. It would be nice to get a fast moving VCA in the equation, for the times when you want a snappy percussion sound. Three SSM2164s could do it.

November 21, 2008

After a month of downtime, I am back in the saddle. I got tonsillitis and had to go to the hospital, then my grandmother died. It's been a bummer, but I've done a little bit of work on a couple modules that I'm ready to show. These pictures are just the panel designs on paper with knobs laying on top, but they give a suggestion of what the finished modules will look like.

The first is a clone of an oldie. I don't care too much for the idea of cloning old modules (I'd rather do something new), but people keep asking me about 258 oscillators. It makes sense that some 200e users would crave the raw analog sound of the classic 200 sound source. There are a couple things that make this a funny one to re-do in modern times.

The first is that it used a µA726 matched transistor pair. That transistor is heated to maintain a constant operating temperature, so the scale doesn't drift as it heats up. This part is long obsolete. Although I could buy NOS ones on ebay, I have decided that this is not the best idea. Since the 259 uses a regular matched pair and a tempco resistor and the 208 switched from a µA726 to a regular matched pair and tempco in one of it's design revisions, I figured it would be ok to do this one with an LM394 and a tempco resistor. I also have redone the PCB so that all the panel controls mount on it. It makes the module much cleaner looking and less prone to failures. If the pots fail 20 years from now and nobody is making the same ones I have used, one could always panel mount some and wire to the PCB like the original design. It could be built up using the original knobs, blue knobs (if I can get the big blue ones!) or the ones Don is using now on the 200e modules.

The second thing is that the 258 has no keyboard input. Don decided to put a fine tune on the left-most CV in instead. I have never cared much for this and have entertained several other options. The first idea was to put a push/pull pot in the left-most position and when it is pulled engage a trimmed CV in rather than the scalable one that is normally there. Grant Richter told me he had done this on an actual Buchla 258. The problem I have with that is the depth of those pots would require really long stand-offs and panel mounted pots. The second idea I had was to put a gray banana jack in the hole where Don put the fine tune control on the left. This would work, but I don't know that I like having a banana jack in the area where the knobs are, call me weird. The final idea I had was to put a toggle switch where the fine tune was. If it's up, the input is trimmed 1.2volts/octave or whatever and if it's down, the CV goes through the control. I have set up my PCB so that the original fine tune control, the banana jack or the switch are all possible. I plan to try them all out and decide which suits best.

Although I did a couple 258 clones a few years back using a CA3080 instead of the discrete transistors in the core, I found the triangle symmetry to be imperfect and the whole exponential converter had to be changed to use a PNP pair, hence this module will be true to the original design using discrete transistors.

The second module I have ready to order parts for is the Quad Voltage Processor I wrote about last month. I thought I could get all the parts between the controls on a single PCB, using SMT, but I found that it was more parts than I anticipated. It is a motherboard with all the panel controls and a second board with the actual circuit. Since this is essentially 2 whole 257 modules, each with an extra CV input, I think that is respectable enough. I'm really looking forward to getting this one built up. This will be a boon to small 200 systems, where it will solve many control voltage situations.

November 3, 2008

I cracked open my first 292c repair last night. I found some interesting things during this repair. First, there are some parts in the actual 292c that are not on the schematic. I probably found the most confusing way to draw this, but I just added it on top of the schematic in red. the cap around IC7 is going from inverting input to output, even though my artwork may look misleading. I'm not sure what the point of that resistor/cap kludge in the middle is. My guess is to roll off the highs (around 24k) a little bit in gate mode only. This makes me wonder why nobody has complained about oscillations in all this cloning that has been going on. Check out the hacked solution to solve the lower gain on the second channel. I don't think that's factory.

Second, the channels don't match! Well, the first one on the left has an error, so that doesn't count, but the second one doesn't have a 100p around IC7 at all!?

I have been way sick for the last week. My tonsils got infected and I haven't been doing any work. I took this shot just for fun. These modules will never live together, but it's a fun hypothetical 12 spacer. Now I just need to make them all work...

November 1, 2008

In the last trade I did, I ended up with 2 modules that seem to be very early examples. The first is a 292B labeled "Prototype" on tha back of the panel. It's written in pencil, funny enough. This one worked well and matches the 292B I already had here.

The front panel was dirty, I had to take all the parts off to clean it up.

The other, a beat up old 207, has "Model" on an orange sticker, stuck to the back of the panel. I believe this means that Don built this up for the assemblers to use a reference while they were hammering them out. This thing had a few broken sliders, so I decided to replace all of them, since I was ordering a set for the 207 I was building from scratch. Synth Restore in the UK has re-manufactured sliders for ARP synths, and these were CTS too, so they work for Buchla stuff. To my horror, the inside of this thing was filthy! Balls of hair at the top and bottom of each slider and crumbs were everywhere.

I took out the sliders and blew out the crap to get started.

The sliders went in really nice and even though the shafts are shorter than Buchla used, they look great. Now, I just have to finish his buddy...